A dialkyltin dialkoxide is very useful as a catalyst such as a carbonate synthesis catalyst, a transesterification reaction catalyst, a silicone polymer or urethane curing catalyst.
As a conventional process for producing a dialkyltin dialkoxide, there is known a method in which a dialkyltin oxide and an alcohol are subjected to a dehydration reaction, and a low boiling component containing water produced is removed from the reaction liquid (see, for example, Patent Document 1: U.S. Pat. No. 5,545,600, Patent Document 2: WO 2005/111049, Patent Document 3: Japanese Patent Application Laid-open No. 2005-298433, Non-Patent Document 1: Journal of Chemical Society, 23 (1971), 3972, Non-Patent Document 2: Journal of the Chemical Society of Japan—Industrial Chemistry, 72, 7 (1969), 1543).
This method using the dialkyltin oxide is presumed to involve an equilibrium reaction accompanied by dehydration as shown in following formula (6):

The above equilibrium is biased overwhelmingly toward the reactant system side, and furthermore the reaction is presumed to include successive dehydration reactions going via a tetraalkyltindistannoxane as shown in formulae (7) and (8) below. To obtain the dialkyltin dialkoxide with a high yield, production is carried out while withdrawing water out of the system from out of the dehydration reaction products, but this is an energetically unfavorable reaction, and hence the dialkyltin dialkoxide is obtained through prolonged reaction at a high temperature (e.g. 180° C.). The following dehydration reaction is carried out, and excess alcohol is removed from the reaction liquid, whereby a reaction liquid containing the dialkyltin dialkoxide is obtained.

On the other hand, it is known that at such a high temperature, a dialkyltin compound is readily thermally decomposed into a trialkyltin compound (see, for example, Non-Patent Document 2: Journal of the Chemical Society of Japan—Industrial Chemistry, 72, 7 (1969), 1543). It is not clear by what reaction the trialkyltin compound is produced, but if it is assumed, for example, that the trialkyltin compound is produced through intramolecular alkyl group rearrangement, then it is presumed that the trialkyltin compound is produced by a disproportionation reaction as shown in following formula (9):

A dialkyltin dialkoxide obtained by a production process using the reaction described above is used, for example, for producing a carbonate through reaction with carbon dioxide (see, for example, Patent Document 2: WO 2005/111049). Thermally decomposed matter is produced in the dialkyltin dialkoxide production process as described above, but moreover it is presumed that in addition to this thermally decomposed matter is also produced in steps in which the dialkyltin compound is heated (e.g. the carbonate production step and a carbonate/dialkyltin compound separation step). Furthermore, it is known that such thermally decomposed matter contains the trialkyltin compound and a high-boiling-point tin component of unidentifiable structure, and that the trialkyltin compound exhibits hardly any activity in the carbonate synthesis using carbon dioxide (see, for example, Non-Patent Document 3: Journal of American Chemical Society, 121 (1999), 3793). In the present invention, the high-boiling-point tin component of unidentifiable structure in the thermally decomposed matter is referred to as a “high boiling deactivated component”. Herein, “high-boiling-point” or “high boiling” means a boiling point at normal pressure higher than 250° C.
The above thermally decomposed matter is a deactivated component that does not exhibit reaction activity in the carbonate synthesis, and moreover, may cause a reduction in the reaction yield or contaminate the product, and hence must be separated out from the dialkyltin compound that is the active component (hereinafter, this component having two tin-carbon bonds on each tin atom constituting an alkyltin alkoxide is often referred to as the “active component”).
The present inventors have previously disclosed an invention relating to production of a high-purity dialkyltin alkoxide (see, for example, Patent Document 3: Japanese Patent Application Laid-open No. 2005-298433). In this document, there is disclosed a process for producing a dialkyltin alkoxide not containing a harmful trialkyltin compound. As a result of their studies, the present inventors have ascertained that such a trialkyltin compound has a low boiling point among alkyltin alkoxide compounds, and hence a high-purity dialkyltin alkoxide can be obtained by removing the trialkyltin compound through distillation. On the other hand, a problem has remained that, of thermally decomposed matter, a high-boiling-point tin component of unidentifiable structure (the above “high boiling deactivated component”) still remains mixed in with the active component.
Moreover, the present inventors have also disclosed an invention relating to production of a carbonate using an alkyltin alkoxide compound containing thermally decomposed matter (see, for example, Patent Document 4: WO 2004/014840). In this document, there is described a method in which, of the thermally decomposed matter, a trialkyltin compound component is separated out by distillation, so as to be prevented from accumulating in the reaction system.
However, for the thermally decomposed matter that is a counterpart to the trialkyltin compound, although a method has been described in which this thermally decomposed matter is precipitated out as solid utilizing the difference in melting point or solubility to the active component, and then separated out from the active component by filtration, so as to be prevented from accumulating in the reaction system, there have been cases in which the active component recovery yield decreases.
Patent Document 1: U.S. Pat. No. 5,545,600
Patent Document 2: WO 2005/111049
Patent Document 3: Japanese Patent Application Laid-open No. 2005-298433
Patent Document 4: WO 2004/014840
Non-Patent Document 1: Journal of Chemical Society, 23 (1971), 3972
Non-Patent Document 2: Journal of the Chemical Society of Japan—Industrial Chemistry, 72, 7 (1969), 1543
Non-Patent Document 3: Journal of American Chemical Society, 121 (1999), 3793
Non-Patent Document 4: Applied Catalysis A: General, 255 (2003), 93